As a heat sink essential for cooling a heating element such as a semiconductor laser element with a high output, a metal body which is constituted by joining a plurality of metal members such as copper-made members and has a structure for circulating a fluid for cooling a heating element in a fluid channel provided inside, is known.
For cooling a semiconductor laser element, normally, a construction in which a plurality of metal bodies are stacked so as to sandwich the semiconductor laser element is employed. In this case, each metal body not only cools each semiconductor laser element but also serves as an electrical conduction path among semiconductor laser elements, so that an electrical potential difference occurs between the circulating fluid and the metal body.
Therefore, when the fluid circulates inside the metal body for a long period of time of about 10000 hours, as shown in FIG. 6, due to the electrochemical reaction process of a portion (a part of the metal body) in contact with the fluid, volume reduction occurs on the anode side of the metal body, and on the cathode side, corrosion products of copper accumulate and adhere thereto.
Such corrosion can be restrained by lowering the conductivity of the fluid. However, the lowering in conductivity (that is, an increase in resistance) increases the activity of the fluid, so that the corrosiveness due to the volume reduction of the metal body increases inversely. In addition, the device for lowering the conductivity becomes large in size and high in cost, so that the lowering in conductivity of the fluid is not suitable for prevention of corrosion. Furthermore, a leakage current flows due to the lowering in conductivity, so that if the fluid circulates during the working period on the order of several years (actual working hours: several ten-thousand hours), the corrosion influence arises.
To prevent such corrosion, it has been attempted to form an isolation layer which prevents contact between the inner wall face of the metal body and the fluid, on this inner wall face without deterioration of the heat exchange performance. For example, in Patent Document 1, a metal such as Au or an Au alloy is employed as an isolation layer. In Patent Document 2, artificial crystals such as artificial diamond and artificial sapphire are employed as an isolation layer.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-273441    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-110186